Clint Eastwood continues the story of “Flags of Our Fathers” with “Letters from Iwo Jima,” but the two movies couldn’t be further apart in tone or ambition. “Iwo Jima” takes the viewer into the heart of darkness watching Japanese soldiers fight the enemy and themselves in a struggle to comprehend duty to their country and their own instincts for survival. It’s moving material, but lacks an editorial refinement and center of gravity that would make it devastating.

“Letters from Iwo Jima” was conceived as a companion piece to Clint Eastwood’s autumn film, “Flags of our Fathers.” Fearing the American story was overshadowing the slaughter at Iwo Jima, Eastwood slapped this film together in a hurry. He shot it after “Flags” had wrapped, kept it at least 95% of the dialogue in Japanese, and pushed the action away from the beach backdrop to the murky tunnels the Japanese soldiers hastily dug as they prepared a defense to fend away the Americans.

Thematically, “Iwo Jima” doesn’t reach quite as high as “Flags,” and those anticipating a parallel war epic are going to be disappointed. This is a somber character study; a gut-wrenching exploration of the bleak Japanese experience during the battle. The picture keeps a low profile, leads with an eye toward grief, and erases the widescreen polish that kept “Flags” at a safe, respectful distance. The economical nature of the production makes it stand apart as something exceptional, and also suppresses it from breaking away and reaching idealistic heights of cultural awareness.

Like a squirmy kid, “Flags” kept wiggling away from the viewer, trying to juggle three complicated stories at once. “Iwo Jima” has the benefit of staying in one place, often quite literally. The action keeps to the paranoia and death in the caves closely, only cutting away for the brief battle sequences and the hushed flashbacks, which present certain characters with backstory on their peaceful lives before the war stole their soul; the “Letters” of the title.

“Iwo Jima” benefits from a tighter focus because, unlike “Flags,” the audience is able to get right into the pocket of danger with the characters. Sympathies are made with the Japanese army, who are fighting the Americans with mere scraps of supplies, almost no men, and not a single place to retreat. These soldiers are marching to their doom, and the script examines the anguish and the comfort in an odd way, of certain death.

There are startling, unsettling images in “Iwo Jima” that are 10x the horrors of “Flags.” One sequence that immediately paralyzes is a brief look at soldiers under strict orders to kill themselves with grenades before enemy forces could capture them. One by one they each pull the pin, grip the device tentatively, and wait for an earsplitting, excruciating death. It’s unbearable to witness.

What Eastwood has achieved here is authentic and, at select times, achingly humane and riveting. It just isn’t efficient. “Iwo Jima” has the tendency to hit the same emotion and story beats over and over, repeating itself into a slumber. The picture slows down to a crawl with every backward step, and it’s hard not to notice this 130-minute film could’ve used another pass with a more courageous editor.

Also of small disappointment is Eastwood’s disinterest in lacing “Flags” and “Iwo Jima” together in small, fringe ways. The director seems adamant that the two pictures stay separate. Eastwood misses out on a chance to bring about a larger canvas of conflict by separating these conjoined twins, and, as affecting as “Iwo Jima” is, it doesn’t always throw the knockout punch the renowned director is intending.